In general, the HDD is referred to as an auxiliary memory device designed to write and read data by rotating a disc-shaped aluminum plate coated with a magnetic substance thereon. Although the HDD has a disadvantage that it cannot be used replaceably with others in accordance with the need, it is widely used in small-sized computers because it is cheap and has a large storage capacity. Particularly, since a small-sized HDD is suitable to be used in personal computers, they are widely used in the field of the personal computers. As the capacity of HDD used in such personal computers, 1 GB-class HDD was basically used in the mid-1990's and 16GB-class HDD has been generalized in the late 1990's. Globally, the storage capacity are increasing 60% in every year and the cost thereof is on the decrease of 12% on a quarterly basis.
Typically, the HDD has phonorecord-shaped, overlapped platters on which concentric circles referred to as tracks are inscribed and data are electronically written into the circles. Briefly, the HDD comprises a hard disc having a plurality of platters 1 stacked thereon, a spindle motor 2, a head 3, head arm 4 and a stepping motor, as shown in FIG. 1.
The platter 1 is made by thinly coating a metal disc with a magnetic substance having magnetism. A high-capacity HDD uses several sheets of platters 1 because of the limitation in the capacity of data that can be written into one sheet of platter 1 and the size of HDD is determined depending on the platter's size and numbers.
The spindle motor 2 rotates at a given speed (for example, 3600 rpm, 5400 rpm, 7200 rpm) when a power is applied to the motor which rotates the platter, and one or more platters 1 are connected to a spindle axis of the spindle motor 2 and thus rotate simultaneously. In order to reliably read/write data, it is important to exactly control rotating rates of the spindle motor 2, among other things.
The head 3 moves horizontally above and below the rotating platter 1 and read/write data on the platter 1. The head arm 4 allows the head 3 to move and adjusts the location of the head 3 in response to receiving commands from a controller chip.
Finally, the stepping motor 5 provides motive power for moving the head 3 to the position of platter 1, and an access time (i.e. the total time taken for the HDD to allocate or transfer data to a memory) is determined depending on a good or bad condition in performance of the stepping motor 5.
Particularly, among the foregoing, the spindle motor to which the present invention applied belongs to a brushless-DC motor (BLDC motor) and rotates the platter 1 by transferring a rotating force to the center of the platter. This motor is widely used as a laser beam scanner motor for laser printer, a motor for floppy disk drive (FDD), a motor for optical disk drive such as compact disk (CD) or digital versatile disk (DVD) and the like.
In machines requiring such high capacity and high-speed driving power as the above HDD, spindle motors adopting a hydro dynamic bearing having driving load (or driving friction) less than a conventional ball bearing type are the tendency of favorite usage.
Herein, since the hydro dynamic bearing basically forms a thin hydro film between a rotator and a stator and supports the rotator by a pressure generated when rotating, the rotator and the stator are not contacted each other, resulting in a decreased friction load. Therefore, the spindle motor of the hydro dynamic bearing type is distinguished from such spindle motor of ball bearing type supporting the shaft by steel or ceramic balls in that a lubricating oil holds the shaft of the motor which rotates the disk by means of the only dynamic pressure.
Further, in case of the spindle motor of the ball bearing type, there are disadvantages that the ball bearing generates noise and vibration due to the contact of component parts, ball and race wheel and particularly the vibration acts as an obstacle factor in promoting the track density of hard disc. In contrast, in case of the hydro dynamic bearing based on a oil pressure force, there is no metal friction and the more a rotating speed is fast, the more stability is improved. Therefore, the bearing has been adopted primarily on the HDD due to less noise and vibration properties thereof.
As such, the internal structure of a conventional spindle motor of a hydro dynamic bearing type consists of a base 26, a sleeve 25, a stator core 27, a shaft 23, a hub 21 and a magnet 22 as shown in FIG. 2.
In the spindle motor, the sleeve 25 is fixedly coupled perpendicularly to the inner side of the base 26 forming an appearance, a stator core 27 having coil wound thereto is mounted on the outer of the upper side of the base 26 and the shaft is rotatably inserted to penetrate through the inner center portion of the sleeve 25. The lower portion of the shaft 23 is rotatably coupled with a disc-shaped thrust bearing 24 together with the shaft 23, the lower end thereof is sealed from the outside by a thrust cover plate, and the upper portion of he shaft 23 is coupled with a cap-shaped hub 21 having its inner opened downwardly. And, the inner side of the end portion of the hub 21 is, attached, at the position facing the stator core 27, with a magnet 22 and an clearance for oil is formed between the outer side surface of the shaft 23 and thrust bearing 24 and the sleeve 25 and filled with fluid substance such as lubricating oil, grease, etc.
Accordingly, if the hydro dynamic bearing type of spindle motor is applied with an external power, the hub 21 and the shaft 23 coupled to the hub is rotated by an electromagnetic repulsive force acting between the stator core 27 and magnet 22.
And, typically, the groove formed in the outer circumference of the shaft 23 is of a herringbone or spiral shape as shown and thus if the shaft 23 is rotated, the oil filled in the oil gap moves towards the center portion of the groove 28 and generates a hydro dynamic pressure to support the shaft 23, thereby supporting the shaft 23 and preventing a dispersion of fluid substance filled in the oil gap.
However, for the spindle motor using hydro dynamic bearing as shown in FIG. 2, the shaft 23 which is the axis of the hub 21 is not fixed, but supported by the fluid substance within the sleeve 25 and thus the shaft 23 needs a relatively large radial hydro dynamic pressure, compared to the case that the shaft 23 is fixed. Accordingly, in case that the length (L1) of the shaft 23 is shortened, it is not possible to obtain a large hydro dynamic pressure and resultantly there exists a disadvantage that it is difficult to manufacture a thin spindle motor.
Further, there is a problem that it is difficult to comply with the requirements for main properties required in the spindle motor for HDD such as a low non repeatable run out (low NRRO), low noise, high speed rotation, high intensity, low power consumption, high confidence, a small size, low dust, low cost and the like.